In fluid drilling apparatus in general, and in particular in apparatus of the type described in Australian patent specification AU700032, the rock through which a bore hole is being formed by fluid jet erosion is often hard and difficult to cut or erode by water jet action.
It is a problem with fluid drilling apparatus of this type that the forward progress of the cutting head is difficult to regulate due to the inconsistent nature of the rock being cut. It is common for the cutting head to be held up in areas of harder rock, causing over reaming of the surrounding rock in this area until the rock in front of the head is cleared sufficiently to enable the cutting head to advance, whereupon the cutting head surges forward resulting in inconsistent and uneven diameter of the bore being cut.
In waterjet drilling practice using a drill similar to that described in Australian patent specification AU700032 the high pressure waterjets cut the rock ahead of the drill forming rock chips called cuttings. The spent jet fluid then flows back along the borehole, firstly through the annulus formed between the body of the drill and the borehole wall and then through the much larger annulus formed between the high pressure supply hose and the borehole wall. The cuttings are carried along in the flow of this spent jet fluid. The volumetric flow rate of the waterjets is constant for a given combination of pump pressure and nozzle diameter, whilst the rate of cuttings produced is determined by the drill penetration rate and the borehole diameter.
In order for the spent jet fluid and the cuttings to flow back through the annular area formed by the body of the tool and the borehole wall a pressure differential is required across the length of the tool. Hence, a higher pressure acts on the front surface area of the drill compared to the back surface area. The magnitude of this pressure differential is determined by the equivalent flow area of the annulus, the volumetric flow rate of the spent jet fluid and cuttings, and the length of the tool body. If the equivalent flow area of the annulus is sufficiently small then the resultant pressure differential is sufficiently large as to create a backward acting force greater than the net forward force created by the retro-jets. This will stop the advancement of the drill, possibly even resulting in the drill being forced backwards. This is referred to as “drill stalling”.
Two separate but related situations can cause the tool to stall. Firstly, if the diameter of the cut borehole is below a critical value, then the tool will stall. Secondly, if cuttings particles larger than the annular relief are generated, they can partly block the annulus region thereby reducing the equivalent flow area causing the tool to stall.
There is also a conflict of requirements in the area of the rotatable nozzle assembly of the fluid cutting head between leaving sufficient clearance for particles of rock eroded by the water jet action to clear the rotating nozzle assembly and be carried rearwardly in the fluid flow, and the necessity to locate the outlet from the high pressure fluid jet nozzles as close to the rock face as possible in order to optimise the cutting force.